This invention relates to an integrated powertrain control for a motor vehicle, and more particularly to an engine control method that limits engine output torque and speed during transmission range changing.
The range selector of a vehicle equipped with an automatic transmission allows the driver to shift the transmission from neutral to the forward or reverse speed ranges. Such range changes usually occur in the context of a so-called garage shift at the initiation of vehicle operation, or when it is necessary to temporarily reverse the direction of vehicle movement during parking, for example. In these situations, the engine speed and torque are typically very low, and the transmission clutch pressures may be tailored to provide a smooth engagement; see, for example, the U.S. Pat. No. 5,046,178 to Hibner et al., assigned to the assignee of the present invention. However, range changes also occur during a rocking maneuver when attempting to free a vehicle that is stuck in snow or soft ground. In this situation, the engine speed and torque may be quite high, to the point of being abusive, and some sort of interventionary control may be needed to prevent damage to the transmission and drivetrain. One example of such a control is described in the U.S. Pat. No. 5,833,572 to Leising et al., where the engine speed or torque is reduced to limit the transmission input speed during the engagement of a transmission range clutch if the time in neutral is short and the engine throttle setting during the neutral condition is above a predetermined value. Alternatively, potentially damaging range shifts can be avoided simply by inhibiting range shifting when the engine speed is above a calibrated value. While such controls may prevent transmission damage, they seriously and unnecessarily limit the rocking capability of the vehicle. Accordingly, what is needed is a control method that limits engine torque in a way that protects the transmission during range changing without unnecessarily limiting the rocking capability of the vehicle.
The present invention is directed to an improved and coordinated engine control for a motor vehicle including an automatic transmission, where the control coordinates engine speed and torque limits during transmission range changing so as to provide both vehicle rocking capability and adequate protection for the transmission. According to the invention, the transmission controller temporarily limits the engine speed to a predetermined value on each transition from neutral to the forward or reverse ranges, and superimposes continuous engine speed and torque limitations when an abusive rocking maneuver is detected. An abusive rocking maneuver is detected when the driver commands at least a specified number of successive range shifts during a predetermined time interval, and the speed and torque limitations are imposed until range shifting is discontinued for at least a calibrated period of time.